Optical sensors are known, consisting of a plurality of photo-sensitive elements, or pixels, able to detect light signals and to transmit them, in the form of electric signals, to a calculator which processes them and obtains images from them which it transmits to display devices; the latter are then able to allow a user to see such images or information deriving therefrom.
Previously, such optical sensors were made using CCD technology (Charge-Coupled Device), which guarantees a very satisfactory image quality in the presence of a well-controlled illumination, but are not able to operate optimally in the presence of a light which is greatly differentiated inside the same scene, that is, with an input signal having high dynamics, up to 150 dB.
CCDs are also not very versatile from various points of view: they cannot easily be integrated with complex pilot circuits in a single silicon support (called microchip), and it is not possible to arbitrarily select a sub-window inside the matrix sensor.
To overcome some of these shortcomings of CCDs, optical sensors have been developed based on the CMOS type silicon technology (Seger, Graf, Landgraf—“Vision Assistance in Scene with extreme Contrast”—IEEE Micro, vol. 13 page 50, February 1993), which offer a good result in very differentiated lighting conditions inside the same scene. This result is obtained by means of a conversion on logarithmic scale of the signal inside the photo-sensitive element or pixel.
Such logarithmic conversion, obtained for example by connecting an MOS type transistor in diode configuration to the photo-sensitive joint, as described in U.S. Pat. No. 5,608,204, suffers in any case from the fundamental disadvantage that it supplies a low definition of the image in the event of low illumination. High resolution images are obtained by means of a linear reading of the photo-sensitive element; this technique, however, has the disadvantage that it does not give the possibility of obtaining good quality images in very differentiated lighting conditions inside the same scene.
The Applicant has devised and embodied the present invention to overcome these shortcomings of the state of the art and to obtain further advantages.